Advances in micromachining technology have given rise to a variety of Micro-electromechanical systems (MEMS) including light modulators for low cost display applications. Such modulators provide high-resolution, high operating speeds (KHz frame rates), multiple gray scale levels, color adaptability, high contrast ratio, and compatibility with VLSI technology. One such modulator has been disclosed in U.S. Pat. No. 5,311,360, titled "Method and Apparatus for Modulating a Light Beam" issued May 10, 1994, by Bloom et al. This modulator is a micromachined reflective phase grating. It consists of a plurality of equally spaced deformable elements in the form of beams suspended at both ends above a substrate thereby forming a grating. The deformable elements have a metallic layer that serves both as an electrode and as reflective surface for incident light. The substrate is also reflective and contains a separate electrode. The deformable elements are designed to have a thickness equal to .lambda./4 where .lambda. is the wavelength of the incident light source. They are supported a distance of .lambda./4 above, and parallel to, the substrate. Thus, when the deformable elements are unactivated, i.e., undeflected, the distance between their top surface and the substrate equals .lambda./2. Thus, when light impinges perpendicularly to the surface of this surface the grating reflects light as a flat mirror. However, when a sufficient voltage (switching voltage) is applied between the deformable elements and the substrate, the resulting electrostatic force pulls a portion of the deformable elements down a distance .lambda./4 toward the substrate, thereby reducing the distance between the top of this portion of the elements and the substrate to .lambda./4. Thus, light reflected from this portion of the deformable elements is out of phase with that from the substrate and a diffraction pattern is formed. Optical systems can intercept the diffracted light with output occurring only when the deformable elements are activated (i.e., pulled down). For display applications, a number of deformable elements are grouped for simultaneous activation thereby defining a pixel, and arrays of such pixels are used to form an image.
U.S. Pat. No. 5,677,783 titled "Method of Making a Deformable Grating Apparatus for Modulating a Light Beam and Including Means for Obviating Stiction Between Grating Elements and Underlying Substrate," issued Oct. 14, 1997, by Bloom et al. discloses a modulator which obviates stiction between grating elements and underlying substrate. One problem with the prior art modulator is that it is activated via an electrostatic force which is nonlinear. Specifically, as the voltage applied to the modulator increases from zero, the activated deformable elements deflect incrementally until they reach approximately 1/3 of their full scale deflection, and then they jump the remaining distance until they impact the substrate. Therefore, when the prior art modulator modulates light, the activated deformable elements contact the substrate and this gives rise to significant stiction problems.
Therefore, a need exists for a modulator in which the deformable elements can be held stationary at any point over the entire range of their motion so that light modulation can occur without the deformable elements contacting the substrate thereby eliminating the stiction problem.